Water-soluble epoxy resin derivatives suitable for use in aqueous thermosetting coating systems, useful for example for electrodeposition onto metals are known. In U.S. Pat. No. 4,331,574, issued May 25, 1982 carboxyl-containing resinous compounds are disclosed which are suitable for making water-dilutable thermosetting coating compositions after at least partial neutralization with a base, which compounds are prepared by a process which comprises reacting a non-acidic hydroxyl-containing resinous intermediate with a polycarboxylic acid anhydride at a temperature up to about 150.degree. C., wherein said resinous intermediate is prepared by reacting below about 150.degree. C. in the presence of an esterification catalyst, in at least one step (a) a diglycidyl ether of a dihydric phenol having a weight per epoxide (WPE) not exceeding 600 with (b) a hydroxylalkane monocarboxylic acid and (c) dimerized unsaturated fatty acids in an epoxy or acidic equivalent ratio of (a), (b) and (c) from x/2/(x-2) to (x+0.1x)/2/(x-2), wherein x=4 for a diglycidyl ether having a WPE of from about 400 to 550 and x= 6 to 8 for a diglycidyl ether having a WPE of from about 170 to 250, said resinous intermediate being reacted with said polycarboxylic acid anhydride in an amount to produce a half-ester having an acid value of from about 5 to 35 mg KOH/g.
Aqueous compositions containing such resinous compounds tend to deteriorate on storage, giving rise to reduced performance when stored compositions are used for electrodeposition and to inferior physical properties in eventual cured coatings. This deterioration is believed to be due at least in part to hydrolysis of ester linkages associated with the parts of molecules in the resinous compounds derived from the monocarboxylic acid component (b).
There has now been discovered a process for preparing carboxyl-containing resinous compounds having advantageous hydrolytic stability.